> > >Semantics
> > >is of course is issue of perspective. What is semantics at one level
> > >is just machinery at another.
> >
> > I couldn't disagree more. My entire professional career has been
> > devoted to showing why this false. I think that you are using
> > 'semantics' in an informal sense which is almost certainly
> > insufficiently precise to support the infrastructure being envisioned
> > for the SW.
>
>Perhaps you could do me a favor. It is my understanding that e.g. HTTP
>URI scheme semantics applicable to the structural components of an 'http:'
>URL are irrelevant and invisible to an RDF processor which is using that URI
>as the identity of a resource within an RDF graph, and in fact, within RDF
>space, that URI is used as an opaque symbol to which is attached semantics
>which is disjunct from any semantics meaningful to or associated with the
>URI Scheme of the URI, such as the semantics of an 'http:' URL.
>
>Now, we have here two functional layers: RDF, and HTTP. A SW agent
>may interact with that URI at either level, and the semantics at
>one level does not have significance at the other.
Ah, I see what you mean, and agree with you. I thought (too hastily -
is there a smilie for eating humble pie?) you were saying something
larger and more, er, philosophical about the nature of meaning.
I wonder if in fact this picture of completely separated layers is
really supportable in the long run, and should be replaced by a more
sensitive account of meaning. But I agree with you that the current
RDF understanding is that, as you say, URI's are simply treated as
opaque symbols at the level of RDF semantics; their only requirement
is that they be globally unique, so that a given URI has the same
interpetation wherever it occurs.
>When applying
>some axiom or inferring some relation, the HTTP semantics are totally
>irrelevant. When dereferencing that URI for perhaps some auxilliary
>knowledge, the semantics that is defined for the URI in RDF-space is
>irrelevant to the HTTP server.
DAML+OIL, the next 'layer' above RDF for some folk, actually does
make some nontrivial use of the HTTP semantics, in a sense, in that
it has ways of using URIs to refer to other sets of axioms and import
them. This is admittedly a rather simple kind of cross-layering
interaction, but it hints at the kind of issues that are going to
arise.
>Now, if my understanding of the division of semantics between functional
>layers in such a context is incorrect, I would very much appreciate
>understanding why.
>
>My discussion of semantic layers was specifically focused on the fact
>that if a QName in an XML serialization is mapped to a QName URI (not
>a URI following the URI Scheme of the namespace URI, as is now the case),
>the structure of the original XML QName remains explicitly defined in the
>resultant QName URI, and hence QName semantics can be applied without
>limitation
>to that URI if and as needed; yet even though the URI Scheme maintains
>the QName structure and hence "preserves" the validity of QName semantics,
>that QName URI does *not* introduce QName semantics into RDF, since all URIs
>in RDF are merely opaque identifiers, to which is attached *additional*
>semantics, and it is only that additional semantics at the RDF level
>that is relevant to RDF and RDF based tools operating within the realm
>of the RDF conceptual graph.
>
>I.e. No URI Scheme can introduce any semantics into RDF. The use of any
>URI Scheme for resource URIs has no relevance whatsoever to semantics
>associated with an RDF graph. Right?
Right, other than ensuring the coherence of the assumption of global
name uniqueness (which I think is a *syntactic* matter for RDF.)
>Secondly, I was referring mostly to semantics associated with ontologies
>and identified by both URIs in the graph and QNames in serializations,
>and not the semantics of RDF itself -- which I see as yet a third
>layer/level
>of semantics that is disjunct from either URI Scheme semantics or specific
>ontological semantics.
>
>I.e. The semantics associated with a particular ontology which is
>represented
>by and processed according to the RDF conceptual model does not add to the
>semantics of the RDF conceptual model, and visa versa. Both are needed, but
>depending on perspective and the level at which a given operation is being
>performed, one or the other may be irrelevant. The semantics that defines
>what
>a resource is, or what a statement is, or the relation subPropertyOf, is in
>no way dependent on, nor modifies in any way the semantics associated with
>a given URI. No?
Er, yes. That is, you are right. I tend to use the term 'semantic' in
what may be an overly narrow sense, arising from my background in
logic and KR. In a world where many of the ideas come from markup
language design, however, one might expect that one layer's semantics
is another layer's syntax, so I should no doubt have been more
careful in interpreting what you wrote.
All this arose from your central technical point about Qnames and
URIs, however, and maybe it would be more useful to get back to that.
I now see your point about the use of a URI scheme, but that still
seems to me to be basically a matter of politics. Calling a
notational convention a URI scheme, as opposed to part of (say) the
RDF spec, doesn't change the fact that it *is* a notational
convention, and that to ensure its proper use requires that a
community of users agree to conform to it. The difference really
seems to lie in a perception of how 'global' the authority is which
specifies the convention, and therefore how acceptable it is that
this authority be allowed to make such a specification, rather than
in any technical difference between a URI scheme and an RDF usage
convention (such as the use of '#').
Being relatively new to the World of the Web, I find all these
conventions about equally arbitrary, and maybe indeed this perception
is naive; though I would submit, politically naive rather than
technically so.
Best wishes
Pat Hayes
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